2. Field of the Invention
This invention relates to the application of a surrounding jacket to a corrugated metal shield in cable manufacture.
2. Description of Prior Art
In the manufacture of telecommunications cable, it is conventional practice to provide a corrugated metal shield, normally steel, around the cable core which comprises a plurality of electrical conductors. In one cable type, a steel shield is tin plated with overlapped longitudinally extending edge regions of the shield bonded together by a soldering operation. To provide a moisture barrier between the shield and a polyolefin cable jacket, a moisture barrier is provided around the shield by a flooding compound before the jacket is extruded onto it. Originally flooding compound was normally asphalt based. However, asphalt flooding compound is brittle under low temperature environmental conditions at -20.degree. C. and this may facilitate crack propagation in the jacket. It tends to embrittle a polyolefin jacket which may then disintegrate at such low temperatures particularly during installation in a manhole when the cable is subjected to twisting, torsional and bending stresses. At high temperatures, asphalt based flooding compounds have presented no particular problem. To overcome the low temperature disadvantage, the asphalt based flooding compounds were replaced by polyisobutylene flooding compounds which do not encourage embrittlement of polyolefin jacket materials. However, at higher temperatures, and within pressurized cables using polyisobutylene flooding compounds, there has been a lack of mechanical integrity in a cable and gas pressures achieved have caused ballooning of the jacket away from the shield if gas penetration occurs through any unsoldered areas at the overlapped edge regions of the shield. In an attempt to prevent such ballooning occurrences, jacket materials were modified to the use of medium density polyethylenes which are also less crack susceptible. However, jackets of medium density material are susceptible to buckling or localized kinking thereby increasing the diameter or width of a cable locally with resultant jamming of cable in ducting through which it is being fed.
The above cable structures using flooding compounds together with their attendant disadvantages were being replaced during the 1970's by another type of cable in which the plating on a steel shield was replaced by a polymeric coat. The polymeric coat was advantageous in that during extrusion of jacket material onto a shield, the polymeric coat was caused to soften and thereby fuse to the jacket material so that a bonding occurred between the shield and the jacket. The heat to activate the bonding was supplied by the jacketing compound itself. The polymeric coating was sufficiently heat insulating to prevent premature solidification of the jacketing compound. This enabled the use of jacketing compounds with very low melt indices,, i.e. in the order of 0.2 to 0.5 g/10 min [as measured by the procedure specified under ASTM D-1238 (condition E)] while such compounds were able to fill completely the corrugations of a metal shield. However, with such a polymeric coat, soldering of the overlapped edges of the shield was impossible and any bonding between the overlapped edges relied upon a softening of the contacting coating layers during extrusion so that the layers became fused together. However, such a bond between overlapped edges of the shield was not very resistant to torsion and bending stresses placed upon a resultant cable whereby shield edge separation could result and the outer edge of the shield could move outwardly and cut through the jacket. This action is normally referred to as "zippering". In an attempt to overcome this problem, plastic filler has been introduced between the overlapped edges of a shield, but this has led to undesirable complications during manufacture.
More recently, polyolefin materials modified with carboxylic acid or anhydride thereof have become available. It has been found that such materials are useful in jackets for cables in that they may bond to a metal (preferably steel) shield without the use of a polymeric coat on the shield. As a result, both a polymeric coast and a flooding compound together with their inherent disadvantages may now be avoided. However, attempts to apply jacketing material employing polyolefin modified with carboxylic acid to metal shields by known extrusion techniques have so far proved to be unsatisfactory. This is because, while the polymeric coat is not required for bonding, the modified polyolefin contacts the metal directly which acts to promote rapid heat transfer from the polyolefin. As a result, the modified polyolefin commences to solidify at its surface too quickly and cannot flow to contact intimately the whole of the corrugated outer surface of the shield. Trapped pockets of air are thus formed at the bases of the corrugations. While bonding has been successful at the positions where the jacketing material actually contacts the surface of the shield, the bonded regions have been weakened by the presence of adjacent non-bonded regions. In such a structure, resultant cracking an disintegration of a jacket could occur upon the application of bending or torsional stresses to the finished cable.